This invention relates to a low noise amplifier in which the deterioration of its S/N ratio due to thermal noise is prevented.
In a conventional non-inversion amplifier, as shown in FIG. 1 its gain is determined by a resistor 2 connected between the inversion input terminal of an operational amplifier 1 and the ground and a feedback resistor 3 connected between the output terminal and the inversion input terminal of the operational amplifier 1, and an offset bias resistor 4 is connected between the non-inversion input terminal of the amplifier 1 and the ground. This resistor 4 generates a thermal noise voltage -en as indicated in an equivalent circuit in FIG. 2. The magnitude of the thermal noise voltage can be expressed by the following Equation (1) according to the Nyquist Johnson's equation: EQU En-.sqroot.4KT.DELTA.fRa (1)
where Ra is the resistance of the resistor 4, K is the Boltzmann's constant, T is the absolute temperature, and .DELTA.f is the band width.
When a signal source 5 is connected across an input terminal a and a grounded input terminal b of the non-inversion amplifier in FIG. 1, the noise voltage -eni across the input terminals a and b is expressed as follows: ##EQU1##
where Rg is the impedance of the signal source. Accordingly, this can be expressed by an equivalent circuit as shown in FIG. 3.
If the signal source 5 is a tape recorder head or a phonograph pickup cartridge for instance, its impedance contains an inductance component or a capacitance component, and accordingly the optimum damping resistance is provided by means of the resistor 4. In the case where resistance Ra of the resistor 4 should be much smaller than impedance Rg of the signal source 5 (Ra&lt;&lt;Rg), the S/N ratio is markedly deteriorated as is apparent from Equation (2).